Re: [RFC PATCH 00/14] iio: buffer: add support for multiple buffers

[Ardelean, Alexandru]

On Sun, 2020-05-10 at 11:09 +0100, Jonathan Cameron wrote:
> [External]
> 
> On Sat, 9 May 2020 10:52:14 +0200
> Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> 
> > On 5/8/20 3:53 PM, Alexandru Ardelean wrote:
> > > [...]
> > > What I don't like, is that iio:device3 has iio:buffer3:0 (to 3).
> > > This is because the 'buffer->dev.parent = &indio_dev->dev'.
> > > But I do feel this is correct.
> > > So, now I don't know whether to leave it like that or symlink to shorter
> > > versions like 'iio:buffer3:Y' -> 'iio:device3/bufferY'.
> > > The reason for naming the IIO buffer devices to 'iio:bufferX:Y' is
> > > mostly to make the names unique. It would have looked weird to do
> > > '/dev/buffer1' if I would have named the buffer devices 'bufferX'.
> > > 
> > > So, now I'm thinking of whether all this is acceptable.
> > > Or what is acceptable?
> > > Should I symlink 'iio:device3/iio:buffer3:0' -> 'iio:device3/buffer0'?
> > > What else should I consider moving forward?
> > > What means forward?
> > > Where did I leave my beer?  
> > 
> > Looking at how the /dev/ devices are named I think we can provide a name 
> > that is different from the dev_name() of the device. Have a look at 
> > device_get_devnode() in drivers/base/core.c. We should be able to 
> > provide the name for the chardev through the devnode() callback.
> > 
> > While we are at this, do we want to move the new devices into an iio 
> > subfolder? So iio/buffer0:0 instead of iio:buffer0:0?
> 
> Possibly on the folder.  I can't for the life of me remember why I decided
> not to do that the first time around - I'll leave it at the
> mysterious "it may turn out to be harder than you'd think..."
> Hopefully not ;)

I was also thinking about the /dev/iio subfolder while doing this.
I can copy that from /dev/input
They seem to do it already.
I don't know how difficult it would be. But it looks like a good precedent.

My concern regarding going to use stuff from core [like device_get_devnode()] is
that it seems to bypass some layers of kernel.
If I do 'git grep device_get_devnode', I get:

drivers/base/core.c:            name = device_get_devnode(dev, &mode, &uid,
&gid, &tmp);
drivers/base/core.c: * device_get_devnode - path of device node file
drivers/base/core.c:const char *device_get_devnode(struct device *dev,
drivers/base/devtmpfs.c:        req.name = device_get_devnode(dev, &req.mode,
&req.uid, &req.gid, &tmp);
drivers/base/devtmpfs.c:        req.name = device_get_devnode(dev, NULL, NULL,
NULL, &tmp);
include/linux/device.h:extern const char *device_get_devnode(struct device *dev,
(END)

So, basically, most uses of device_get_devnode() are in core code, and I feel
that this may be sanctioned somewhere by some core people, if I do it.
I could be wrong, but if you disagree, I'll take your word for it.

I tried using devtmpfs_create_node() directly, and it worked, but again, doing
'git grep devtmpfs_create_node'

drivers/base/base.h:int devtmpfs_create_node(struct device *dev);
drivers/base/base.h:static inline int devtmpfs_create_node(struct device *dev) {
return 0; }
drivers/base/core.c:            devtmpfs_create_node(dev);
drivers/base/devtmpfs.c:int devtmpfs_create_node(struct device *dev)
drivers/s390/char/hmcdrv_dev.c: * See: devtmpfs.c, function
devtmpfs_create_node()

Most calls are in core, and the one in hmcdrv driver isn't convincing enough to
do it.
In fact, some may consider it dangerous as it allows drivers/frameworks to use
it as precedent to do more name manipulation.
Again, if you guys say that this would be acceptable, I can use
device_get_devnode() and other stuff from core.


> 
> Do we want to make the naming a bit more self describing, something like
> iio/device0:buffer0?  Given the legacy interface will be outside
> the directory anyway, could even do
> 
> iio/device0/buffer0 with link to iio:device0
> iio/device0/buffer1 with no legacy link.
> 
> Ah, the bikeshedding fun we have ahead of us!

This may also depend on how much code it takes to implement these many levels of
folder hierarchy.

> 
> I think this set is going to take too much thinking for a Sunday
> so may take me a little while to do a proper review...
> + I have a few other side projects I want to hammer on today :)
> 
> Jonathan
>